In the production of silver halide emulsions, particularly during a precipitation step or a physical ripening step, it is common to add a compound which adsorbs onto surfaces of silver halide crystals and inhibits growth of the crystals (hereinafter simply referred to as inhibitor).
For example, an inhibitor is added in the production of silver halide light-sensitive materials requiring high resolving power, such as emulsions for IC dry plates, in order to make silver halide grains finer or to prevent formation of large grains which is likely to occur during a precipitation step.
The so-called daylight silver halide light-sensitive materials that can be handled under a bright indoor light are frequently employed for printing for the sake of improving the working environment. In this case, light-sensitive materials consisting mainly of silver chloride are produced in order to prevent increases of fog under the indoor light. Here, also, it is necessary to add an inhibitor to achieve fineness of grains and to prevent formation of large grains.
For obtaining a mono-dispersed silver halide dispersion having a narrow grain size distribution, it is necessary to apply a so-called controlled double jet (CDJ) method in which a silver halide emulsion is prepared while maintaining the silver ion concentration constant.
Since presence of even a trace amount of such an inhibitor can generally suppress formation of twins to a significant degree, and a mono-dispersed silver halide emulsion can easily be formed even under conditions of a considerably high pAg value, e.g., higher than 8.0 at 80.degree. C., addition of the inhibitor makes it easy to obtain a mono-dispersed silver halide emulsion without using a CDJ method. It is, as a matter of course, also possible to apply a CDJ method in the presence of the inhibitor to thereby obtain a mono-dispersion having a further narrowed size distribution.
Further, as is known, e.g., based on Journal of Photographic Science, Vol. 21, p. 39 (1973), Japanese Patent Publication No. 42737/80, etc., addition of an inhibitor makes it possible in some cases to form silver halide grains having a specific crystal habit normally difficult to produce, for example, silver chloride grains or silver chloro-bromide grains having a (111) face (regular octahedrons) or a (110) face.
Furthermore, in the production of silver bromide or silver iodobromide crystals, if a mercapto-containing inhibitor is added and the silver ion concentration is kept at a certain constant level in the course of grain growing, such crystals having (110) faces (rhombic dodecahedron) in addition to (100) and (111) faces obtainable by an usual CDJ method can also be formed.
Moreover, when a silver halide emulsion is chemically ripened by sulfur sensitization, noble metal-sulfur sensitization, or the like in the presence of an inhibitor, chemical sensitization is generally suppressed, but under a specifically designed condition, such as an increased amount of sensitizer, advantages such as that contrast is increased can be attained in some cases, although the relative sensitivity is reduced.
Inhibitors that are mainly employed in the above-described situations are classified into two groups; one of which are compounds containing a nitrogen atom capable of forming crystals with silver ions and thereby adsorbing onto silver halide crystal surfaces, including benzotriazoles, benzimidazoles, hydroxytetraazaindenes, purines, etc.; the other group are compounds containing a sulfur atom capable of forming a bond with silver ions and thereby adsorbing onto silver halide crystal surfaces, including mercaptotetrazoles, mercaptotriazoles, mercaptothiadiazoles, benzothiazole-2-thiones, etc.
These inhibitors are, however, compounds that are generally known to act as antifoggants or stabilizers. Therefore, if the inhibitor is used in the preparation of silver halide emulsion grains and remains unreacted in the emulsion after washing, the residual inhibitor considerably inhibits the subsequent chemical sensitization with a chemical sensitizer and also development to the ultimate disadvantages of serious reduction of photographic density or photographic sensitivity, thus makes the product unemployable.
Further, as described above, the use of the inhibitor during chemical ripening is accompanied by a problem that sensitivity is remarkably reduced despite of improvement in fog or gradation.
An additional problem of the inhibitor is that adsorption of spectral sensitizing dyes to the silver halide grains is greatly inhibited.
Under these circumstances, it is necessary to remove the inhibitor used in the preparation of silver halide emulsions from the emulsions or inactivate it, at least by the time of coating the emulsion.
Among various inhibitors, those containing a nitrogen atom to form a bond with a silver ion can be removed from silver halide emulsions by washing the emulsion with acids or aqueous solutions of halides, since adsorption of the inhibitor to silver halide crystals can be markedly weakened by either lowering pH values or heightening pAg values.
To the contrary, it is almost impossible to weaken or eliminate the effect of the inhibitors containing a sulfur atom capable of bonding or adsorbing to silver ions by changing pH levels or using aqueous halide solutions due to conspicuously stronger bonding or adsorption of these inhibitors to silver ions.
The sulfur-containing inhibitors are suited for the purposes of fining grains, inhibiting formation of large grains, controlling crystal habits, and the like, due to their strong bonding force, but, in turn, they exhibit high inhibitory activity on development and also on adsorption of spectral sensitizing dyes, resulting in great deterioration of photographic properties. Therefore, it has been keenly desired to develop a method for reducing or removing the effects of these sulfur-containing inhibitors, to thereby eliminate the above-described various disadvantages that have been encountered in practical use.